Mean cell volume (MCV) of the red blood cells, red blood cell concentration, also commonly referred to red blood cell count (RBC), red cell distribution width (RDW) of a blood sample are directly measured red blood cell parameters by cell counting and sizing on various hematology analyzers. These parameters are measured with a whole blood sample substantially diluted in an isotonic blood diluent, using direct current impedance (DC) or light scatter measurements in a non-focused flow aperture or a focused flow cell. On these hematology analyzers, hematocrit (Hct) is a derivative parameter of RBC and MCV (Hct=RBC×MCV/10).
For the normal population, MCV has a mean value of 90.3±9.6 fl and RBC has a mean of 4.3×1012/l for adults. The newborn babies have increased MCV, i.e., 101±13. Many diseases, such as iron deficiency, Hgb S-alpha or beta thalassemia, Hgb H, folate or vitamin B12 deficiency, sickle cell anemia, immune hemolytic anemia, preleukemia, cause abnormal MCV values, either higher or lower than the normal value. Similarly, many diseases cause abnormally low or high RBC. These, in turn, cause abnormal hematocrit. Therefore, MCV, RBC, and Hct are clinically important parameters, and the accuracy of the measurements of these parameters is essential to clinical diagnosis.
In a normal peripheral blood sample the red blood cell concentration is about 900 times higher than the white blood cell concentration. The contribution of white blood cells to the red blood cell measurements, such as MCV, RBC and Hct, is negligible, although the white blood cells are present in the sample mixture used for the red blood cell measurements. However, when the white blood cell concentration is very high, for example, 200,000/μl to 500,000/μl, the contribution of the white blood cells to the measurements of red blood cell parameters is no longer negligible.
Among the white blood cell subpopulations, the lymphocyte population can interfere with the MCV measurement, because the size of lymphocytes is the closest to the size of red blood cells. The red blood cells have an average cell volume of 90 fl, and the lymphocytes have an average cell volume of 220 fl. Typically, the red blood cells are measured in a range between 0 to 360 fl, and the MCV is calculated in a range from about 30 fl to 360 fl. The other white blood cell subpopulations, such as monocytes and granulocytes are much larger, and they are outside the dynamic range of the red blood cell size measurement, therefore, do not interfere with the MCV calculation.
However, all white blood cell subpopulations can contribute to red blood cell concentration measurement when the white blood cell concentration is substantially higher than the normal value. The interferences from the white blood cells can generate erroneous MCV, RBC and Hct values, and cause confusion or difficulty for diagnosis.
Currently, the clinical practice in handling a high WBC sample is to manually dilute the whole blood sample to reduce the white blood cell concentration down to less than 130,000/μl. Such a dilution can reduce white blood cell interference to hemoglobin measurement by reducing the turbidity caused by the cellular particles. However, the dilution does not change the ratio between the red blood cells to the white blood cells. Upon dilution, the red blood cell concentration in the diluted sample mixture is also reduced. To maintain the measurement accuracy, the hematology analyzers typically extend the count time to ensure sufficient number of events to be counted in the measurement. Since the ratio between the red blood cells to the white blood cells remains the same, the interferences of white blood cells to MCV, RBC and Hct measurements in the diluted sample mixture remain the same.
U.S. Pat. No. 5,656,499 (to Chupp et al) teaches a method of calculating MCV by setting a left and a right discriminants for red blood cell population on the red blood cell histogram. More specifically, the method first determines the mode of the red blood cell histogram, then on either side of the mode, the first bin (or channel) with a population less than 4% of the population of the mode is identified. These channels on the two side of the mode are defined as the discriminants, and only populations between them are used for calculating MCV. Therefore, Chupp et al define the MCV in their method with the equation of MCV=(mean of histogram between discriminants)×(0.8 fl per bin)×(calibration factor). For the RBC count measurement, Chupp et al teach to identify a zero count bin on the left of the red blood cell mode and set it as the count threshold, and the values greater than this threshold are considered to be red blood cells. Chupp et al's method for MCV calculation has disadvantages. When the lymphocytes and red blood cells have a substantial overlap, such as in the situations of small lymphocytes, or leukocytosis with macrocytic anemia, this method could include the lymphocytes between the discriminants, therefore, report erroneous MCV.
It is apparent that there is a need for prevention and correction of white blood cell interferences to the above discussed red blood cell measurements on automated hematology analyzers.